Captured Hollowing Rig

Updated 26 September 2014

A couple weeks ago I started putting together a captured hollowing rig. I had seen one demonstrated a last year at our turning club and even had a chance to operate it for a few minutes.
I have been doing more hollowing lately and had some failures. And, at least one of my successes was a qualified success in that the wall thickness of the turned piece was pretty thick and the piece was too heavy when it was completed.

There are a number of types of systems out there but right or wrong, I decided I could build the captured style.

There are a number of descriptions of building rigs on the internet. In none of them did I find a materials list, so here is mine:

Qty/Length

Description

Purpose(s)

72 inch

3/4 round cold rolled steel bar

this is divided into the following:

-20 inch

straight handle piece

-18 inch

boring bar

-34 inch

piece to bend into a D for the handle

72 inch

1x1x1/8 steel angle iron

cut in two for the backrest

12 inch

1x1x1/8 steel angle iron

Welded onto the upright support and the lower backrest angle is bolted to it

9 inches

1.25 inch square x 0.120 wall tubing

the (lower) upright on the backrest support – welded to plate (below)

9 inches

1 inch round x 0.120 wall tubing

the (upper) upright on the backrest support. Slides up and down in the square tubing to adjust backrest height

12 inches

6×1/4 Steel Plate

the backrest support (the piece that clamps to the ways)

3 to 4 inches

1-1/4 OD tube with 1/4 inch wall thickness

the coupler between the handle and the boring bar

40 inches

of 3/4-inch tubing

the laser arm and upright

1

laser pointer

Guide cutting tool and control wall thickness

12 inches

3/8 inch rod

this is cut into 3 inch pieces to mount cutters

2

3/16 inch square tool bits

Cut wood

1

Bakelite(?) Knob
from Ace Hardware w/ 3/8×16 thread

Adjust backrest height.This could be made form a 38×16 bolt with a shop-made nut or a bit of rod welded to the head, but this was easier and looks nice.

1

3/8×16 NC Nut

Weld over hole in upright to thread Knob into

2 inches

1-inch round bar

to be cut rounds to make swivels

2

5/16 x 3-1/2 inch carriage bolts (threaded to head)

Supports at ends of backrest angles

2

5/16 NC Nuts

Supports at ends of backrest angles

8

5/16 flat washers

Supports at ends of backrest angles

4

5/16 NC Coupler Nuts

Supports at ends of backrest angles

6

1/4x20x1/4 Set Screws

Retain boring bar into coupler and swivel mounts into boring bar

2

10-32 Button Head Capscrews

Retain swivels to swivel mounts

2

Washers for the 10-32 Button Head Capscrews

Retain swivels to swivel mounts

1

5/15 NC T-nut – brad retained style

For the backrest clamp.

1

5/15 NC x ?? Capscrew (bolt)

For the backrest clamp. The length of this bolt depends on the depth of the ways on your lathe. I used a 1-1/2 inch bolt which is plenty for my lath as the ways are only 1/2-inch deep.

4

Miscellaneous hardwood scraps

Laser mount and fittings

Tooling:

Requires access (or friends with access) to the following tools:

Tool

Purpose

Metal Lathe

Drill hole in end of boring bar

Drill out 3/4 ID Tube for Coupler

Face off swivels (mounts for cutters)

Drill Press

Drill holes for setscrews in coupler and boring bar

Drill holes for retaining screw for swivel

Drill hole for safety pin in back of D-handle

Drill holes for laser tube mounts

Drill holes for clamp in upright for backrest support

Drill holes to bolt lower backrest angle to upright

Metal Mill or milling attachment for metal lathe

Mill slots in swivels for cutting tools (Swivel can be purchased from Monstor Tools or Lyle Jamiesen)

Taps – 1/4×20 and 10-32

Tap set-screw holes and swivel mounts

Welder

Weld D-handle

Weld backrest upright to plate

Weld backrest upright to backrest support angle

Weld coupler to D-handle

OxyAcetylene torch or equivalent

Heat the 3/4-inch bar to make the D-handle

Heat the 3/8-inch bar to bend angled cutter mounts

Angle grinder

Any time I weld I need a grinder

Sawzall with metal cutting blade

Quicker and easier than hacksaw

3/4 to 1/14 wide Mill File

Smoothing, dimensioning, fitting…

3/4 metal drill bit

Drill/ream out inside of 1-1/4 OD coupler to allow Boring bar to slip into it. You may also be able to use this to make the laser tube fittings

Process:

Here is how I did it:

Cut the 3/4-inch round bar into the three pieces listed above.

See step 5 before starting this step. Cut one end of the boring bar at 45-degrees.

Drill the center of the other end of the boring bar to 3/8-inch – I was able to do this on my Chinese 9×20 lathe. This could be done on a drill press.

Drill a 3/8-inch hole at 45-degrees through the bar on the 45-degree end. The hole should be parallel to the cut off end of the bar and should be centered 9/16-inch from the end of the bar (easier said than done).

Drill and tap one setscrew hole in the 45-degree end of the boring bar to hold the cutter in the boring bar. This should run right down the center of the bar. You may want to drill the hole before you cut the 45-degree angle on the end of the bar.

Drill and tap two setscrew holes into the end of the boring bar with the end-drilled 3/8-inch hole. These hold the cutter / swivel mount in the boring bar.

Drill out the 1-1/4 inch coupler tube with a 3/4-inch bit so the 3/4-inch CRS steel boring bar will slip into it.

Drill and tap two setscrew holes in the coupler to hold the boring bar in place.

Bend the D-Handle for the handle of the boring bar. (I had a friend with a OA torch do this. He also had a bending mandrel to make a really nice looking bend)

Cut the 3/8-inch diameter round bar into 3-inch lengths. Put two of the four aside for future projects.

Grind flats on one end of two of the 3/8-inch round bars.

Bend one of the 3/8-inch round bars to 45-degrees. I did this heating it to red hot with a MAPP torch.

Drill and tap the flats of the 3/8 round bars to 10-32 thread to hold the swivels for the cutters.

Cut and true 3/4-inch rounds to 1/4-inch thickness (length) for the swivels.

Drill 0.207 inch holes in the center of each swivel.

Mill a 3/16-inch slot for the cutter across each swivel. This slot can break into the center hole. The depth of the slot should be 1/32 to 1/64 inch shy of 3/16 inch.

Cut the 72-inch angle iron into two 36-inch pieces.

Measure, mark, centerpunch and drill a 5/16-inch hole in each end of each of the 36-inch angle irons. Drill the hole in the cente of the exterior flat of the angle iron (i.e. 1/2-inch from the long edge). I clamped them together “back-to-back” so the holes would line up perfectly. Also, for all the angle-iron drilling, I first pilot-drilled each hole to 5/32 (could be 1/8 or 3/16) before drilling it out to 5/16.

Measure, mark, and drill a 5/32-hole 1/2 (to 3/4) inch in from each end of the 12-inch angle iron. Center the hole on the wide flat (outside) of the angle. Be sure to drill the holes on the same angle on each end.

Mark three positions on the 36-inch angle iron where the 12-inch angle can bolt to it. Clamp the 12-inch angle to the 36-inch angle in each of those positions and use the 5/32-inch holes in the 12-inch angle to drill 5/32-inch holes in the 36-inch angle. Drill a total of 6 holes in one of the 36-inch angles. This will be the bottom angle. Mark and drill the holes such that the three positions are 1) offset 12-inches to the left 2) offset 18-inches to the left and 3) offset 12-inches to the right. These six holes will be in addition to the two holes drilled in step 18. These six holes will be on the same angle face as the holes drilled in step 18.

Measure, mark and drill a hole in the 1-1/4 inch square tubing for the height adjustment knob. The size of the hole is dependant on the thread size of the knob. I used a commercial knob so my hole size is 3/8-inch. This hole can be drilled slightly over sized if desired. Place the hole about two inches from the top of the tube and in the center (side-to-side) of the tube as it will be clamping a round tube.

Weld a 3/8-inch NC nut over the hole drilled in the tubing.

Drill a 3/8 to 1/2 inch hole in the center of the 6x12x1/4 plate, two inches from the end.

Prepare and weld the 1-1/4 square tube to the plate. Be sure to square the tube to the plate in two axes, fore-and-aft and side-to-side.

Weld the 12-inch angle iron to one end of the 1-inch round tubing.

Use two all-thread carriage bolts (or similar) to hold and space the two 36-inch angle irons. I used four coupler nuts to clamp the angle to the head of the bolt and to the nut on the other end – the 5/16-inch coupler nuts take a 7/16-inch wrench and stand off the angle a little bit to make for convenient adjustments.

Use a piece of quality hardwood for the clamp under the ways for the backrest. Check to make sure it will slide underneath the ways of your lathe with no interference. Drill a hole in the center of this block, large enough to insert the T-nut.

Cut notches in the side of the hardwood to fit your lathe.

Drill out the brad holes in the T-nut and use small sheet metal or wood screws to fasten the T-nut to the block.

Turn a knob for the bolt that you are using for the backrest clamp.

Assemble and test.

Above – a close-up view of the backrest – shows the adjustment knob, and also the clamp knob. Also shows how the upright is welded to the backrest support angle, and how that angle is bolted to the bottom angle of the backrest. The two washers between the backrest support angle and the bottom backrest angle are to level the backrest due to a poor job of welding.

Above – shows how the two backrest angles are fastened together. I used bolts that were threaded to the head. I ground the corners off the squares on the carriage bolt heads and capture each angle between either the head or a standard nut and the coupling nuts. This reduces the need for welding, plus lets you adjust the fit of the angles to the D-Handle.

Above pictures shows how the upright is welded to the plate and also how the adjustment knob fits into the upright. IF I do another one I will use round tibing for the inside upright for two reasons. It will make it less critical to have the upright square to the plate and also reduce the need to accomodate the seam inside the squre tube for telescoping action.

Above shows how the T-nut is retained to the backrest clamp. Also shows shop-made knob for the backrest clamp.

Above shows what I call the cross-clamp for the laser upright. It is made from an ash block approximately 2 x 3 x 2-1/2. The clamp is made by drilling a 3/4-inch hole all the way through the block and then drilling a hole into the block until it meets the through hole. Then a blind hole is drilled for the upright. I then sliced the block on the bandsaw through the centerline of the through-hole and the cross hole. I drilled 4 pilot holes in the bottom of the block and used wood screws to clamp the block to the D-handle.

At the top of the upright is what I call a T-Clamp. This piece is made from Siberian Elm. The screws you see are blunted wood screws used for set screws in the event there is any movement within the clamp.

The above is not a terrific picture. This is where the laser is mounted to the end of the laser arm. I will very likely be rebuilding this piece. For now, the hole for the laser is countersunk very slightly at the top of the hole. To turn the laser on I push it down so the large diameter part of the hole presses on the button for the laser.